Lugged vehicle tire, and method of building same

ABSTRACT

The tire comprises a preformed carcass having a plurality of precured lugs secured to its circumferential surface by a thick layer-like mass of readily cured cushion gum material. The carcass may be of new construction but would usually be obtained by buffing the circumferential surface of a used and worn lugged tire to render the same substantially smooth and even. The lugs are separately formed by simultaneously molding and precuring retread rubber under high pressure and uniformly-applied heat, so as to possess high strength, density and cut and abrasion-resistance. The carcass, cushion gum and lugs are cemented and stitched together at ambient temperature and in a manner minimizing oxidation at their confronting surfaces. When the circumferential surface of the tire carcass to which the lugs are secured possesses a significant curvature in the width direction thereof, the exterior surfaces of the lugs may be and preferably are provided with stress-relieving grooves in their outer surfaces. The aforesaid assembly is then heated within a steam chamber under differential-pressure conditions for only the limited time necessary to vulcanize the cushion gum material and thereby permanently unite the components.

BACKGROUND OF THE INVENTION

This application is a division of application Ser. No. 509,793, filedSept. 27, 1974, and issued Mar. 23, 1976, as U.S. Pat. No. 3,945,417which was a continuation-in-part of application Ser. No. 449,917, filedMar. 11, 1974, and now abandoned; which was a continuation-in-part ofapplication Ser. No. 366,101, filed June 1, 1973 and now abandoned.

This invention relates to vehicle tires of the lugged type, and morespecifically relates to an improved lugged tire and to a method for itsformation, which method is particularly but not necessarily exclusivelyadapted for the rebuilding of used lugged tires.

In addition to other utilizations, lugged tires are customarily employedupon many kinds of tractors, earth-moving machines and otheroff-the-road vehicles. Such tires are manufactured in a wide variety offrequently extrememly large sizes, and the projecting lug portionsthereof are subjected during use to not only severe abrasive andcompressive forces, but also to shearing forces of very high magnitude.The useful life of the tires, as heretofore built, has tended to berelatively short. Attempts to salvage worn lugged tires by rebuildingthe same have not solved the durability problem, and have met with otherdifficulities as well.

The rebuilding of lugged tires in the traditional manner, by vulcanizingand shaping a "camelback" of retread rubber material in situ upon thebuffed carcass, requires a substantial capital investment in large moldsand materials of various sizes, and also possesses other disadvantagesheretofore recognized in the art. The fabrication and precuring ofcamelback material would still require a large capital investment inmolds and/or materials, since each such camelback could be used for onlya tire of one particular size, and the difference in size between therelatively massive lug portions and the remaining portions of thematerial would make undercuring of the former or overcuring of thelatter, with resulting detrimental effects in either case, hightlylikely. Alternative approaches to the problem of rebuilding lugged tiresare disclosed in Rawls U.S. Pat. Nos. 2,474,013 and 2,611,411, and RaganU.S. Pat. Nos. 3,511,291 and Re. 27006. In accordance with the Rawlsapproach; discrete replacement lugs formed in special shapes of uncuredrubber are temporarily secured to the worn and buffed lugs of the usedtire by a thin coating of cushion gum material and stitching. Theassembly is then heated within a stream chamber to vulcanize both theuncured lugs and the cushion gum material and to unite the components.Although also employing discrete replacement lugs, the Ragan patentsstate that the use of cushion gum material in a relugged heavy-duty tireproduces a "weak join" which will result in separation of the added lugsfrom the tire carcass under the high stresses imposed thereon duringuse. The Ragan patents therefore alternatively teach the rebuilding of alugged tire, totally devoid of cushion gum material, by temporarilysecuring a sheet of uncured retread rubber to the buffed crown surfaceof the tire carcass, then temporarily securing extruded lugs of the sameuncured retread rubber material to the aforesaid sheet, and thereafterheating the assembly in a stream chamber for the length of time requiredto vulcanize the uncured retread rubber material of both the sheet andthe extruded lugs. The aforesaid approach is believed to have certaininherent disadvantages. To obtain satisfactory adherence between theretread rubber material of the sheet and of the replacement lugs, suchmaterial normally must be in a heated condition during the assemblyoperation. In addition to impeding their handling, this requires eitherthat the components be assembled immediately after their formation,while they still retain the heat imparted to them during such formation,or that they be subsequently re-heated. Secondly, since the finalvulcanization step must produce curing of both the sheet of uncuredrubber material and the lugs of the same uncured rubber material, andsuch material is relatively massive and cures at only the customaryrelatively slow rate, the final vulcanization of the assembly requiresconsiderable time to effect. This, in turn, severely restrictsproduction, or requires the provision of a large number of expensiveheating chambers. Additionally, undercuring of the lugs or overcuring ofthe sheet and heat-deterioration of the tire carcass may well result.Thirdly, the replacement lugs cured in situ upon the used tire havesignificantly less density, strength and cut and abrasion-resistancethan would lugs which were precured under conditions of high pressureand more even application of heat. A rebuilt tire wherein the lugs arecured in situ upon the carcass therefore possesses at best only thelimited durability of the original tire, and is also subject to possiblepremature failure arising from the prolonged reheating of the carcass.

SUMMARY OF THE INVENTION

The lugged tire of the present invention may be formed by utilizing anew tire carcass having a substantially smooth outer circumference or,as would more normally be the case, by employing the carcass of a usedlugged tire and rendering its circumferential surface substantiallysmooth by grinding and/or buffing operations. In either case, there areprovided a plurality of lug components which have at any convenientprior time and place been formed by simultaneously molding and curingretread rubber in a mold under high pressure and subtantially uniformlyapplied heat. Relatively thick layers of cushion gum material capable ofrapid curing and having good flexibility both before and after curingare secured upon the prepared circumference of the tire carcass, andupon the base surfaces of the precured lugs. The precured lugs bearingbase layers of cushion gum material are then secured to the cushion gumlayer upon the carcass circumference. Each of the foregoing assemblysteps are preferably performed by cementing and stitching procedurescarried out while the components are unheated and their confrontingfaces are substantially free of surface oxidation. If thecircumferential surface of the tire carcass employed in the assembly hassubstantial curvature in its width direction, the precured lugs thereonare preferably provided one or more transversely extending grooveswithin their outer surfaces, which groove or grooves tend to relieve thestresses which might otherwise be present within the lugs due to theirforced-conformity to the curvature of the circumference of the tirecarcass. Each such groove may conveniently be formed by engaging theexterior surface of the lug-carcass assembly with a heated grooving ironor similar tool while rotating the assembly about its axis. Followingthe aforesaid assembly operations, and the grooving step if such isperformed, the entire assembly is placed within a steam chamber whereina differential external-internal pressure is maintained thereon andwherein the assembly is heated for only the relatively brief period oftime necessary to vulcanize the cushion gum material and therbypermanently unite all components. Since the time required forvulcanization of the cushion gum material is considerably less than thetime which would be required for vulcanization if such material and/orthe lug components were formed of uncured retread rubber,heat-deterioration of the carcass component is avoided and goodoperational efficiency and economy are realized. Also noteworthy in thelatter regards is the fact that the various components need not beassembled immediately after their formation or be in a heated conditionwhen secured to one another. This not only simplifies and facilitatesassembly of the components, but moreover permits the lug components (aswell as the cushion gum material) to be wholly or partially formed inadvance, and then held in inventory until needed. To minimize the amountof inventory required for a small tire rebuilder to practice theinvention, the retread rubber may be and preferably is molded intoelongated bars having the desired cross-sectional shape and from whichlugs of the particular length required for the building or rebuilding ofa specific tire may thereafter be severed during or at any convenienttime prior to their use. The precuring of the retread rubber whichoccurs during molding of the elongate bar is carried out at highpressure in a mold capable of applying heat substantially uniformly tothe material therewithin, so as to impart high high strength, densityand cut and abrasion-resistence to the retread rubber of the precuredlugs.

The lugged tire of the present invention comprises quite dense, strongwear-resistant precured lugs which are bonded to the substantiallysmooth circumferential surface of a new or used tire carcass by arelatively thick layer-like mass, as opposed to a thin coating or sheet,of flexible cushion gum material. In the latter connection, thelayer-like mass of cushion gum material upon the finished tire has aminimum thickness of at least approximately 3/8 to 1/2 inch and may begreater when the lug thickness is in excess of 31/2 inches, as opposedto the 1/16 inch thick sheet of cushion gum material customarilyemployed in association with the rebuilding of non-lugged tires. When aninterconnecting cushion gum mass of the aforesaid thickness is employed,the lug components of the present tire do not tend to be torn away fromthe carcass by the high shear and other forces imposed thereon duringuse of the tire, notwithstanding the fact that the surface bondingcharacteristics of the thick mass of material are no greater per se thanwould be possessed by a thin coating of the same material. While thereasons for this benefical result are not definitely known, apparentlythe thick mass of cushion gum material permits some deflection of thelugs under the impetus of the shearing forces exerted thereon, and/oritself absorbs or dissiplates an apppreciable part of such forces ratherthan transmitting all of them to the relatively-weaker material of thetire carcass.

In any event, the aforesaid attributes in conjunction with the highstrength, density and general wear-resistance of the precured lugsconstituting its primary ground-engaging components, cause the tire ofthe present invention to be exceedingly durable in use. A luggedconstructed in accordance with the invention possesses a useful liftapproximately twice as great as that of lugged tires heretoforeemployed, which consideration is particularly significant in view of thefact that tires of the type in question can normally be rebuilt only asingle time before they must be discarded.

DESCRIPTION OF THE DRAWINGS

Other features and benefits of the invention will be in part apparentand in part pointed out hereinafter in the following description ofillustrative embodiments thereof, which should be read in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of a method of forming a luggedtire illustratively employing a used tire carcass, in accordance withthe invention;

FIG. 2 is a perspective view of a lugged tire constructed in accordancewith the present invention;

FIG. 3 is an enlarged fragmentary view, partially in section andpartially in elevation, taken generally along the line 3--3 through thetire of FIG. 2;

FIG. 4 is a fragmentary perspective view, partially schematic and invertical section, of a mold for simultaneously molding and precuring theretread rubber of the lug components employed in the method and tire ofthe invention;

FIG. 5 is an enlarged fragmentary perspective view, partially brokenaway, of a lug and the carcass prior to their assembly with one anotherbut subsequent to the provision of layers of cushion gum materialthereon;

FIG. 6 is an enlarged fragmentary sectional view taken generally alongthe line 6--6 through the tire of FIG. 2;

FIG. 7 is a fragmentary elevational view of a lugged tire constructed inaccordance with the invention and employing a carcass having itscircumference curved in the width direction; and

FIG. 8 is a fragmentary view, partially in section and partially inelevation, taken substantially along line 8--8 through the tire of FIG.7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring more particularly to the drawings, the tire-building methoddiagrammatically illustrated in FIG. 1 generally comprises preparinghighly durable lug components formed of precured retread rubbermaterial; providing a new or used tire carcass having a buffed andsubstantially smooth circumferential surface; securing upon thecircumference of the tire carcass and upon the buffed bases of theprecured lugs relatively thick layers of tacky and readily-curablecushion gum material; assembling together the precured lugs and tirecarcass having thereon the aforesaid layers of cushion gum material;grooving, in some instances, the outer surfaces of the lug components ofthe assembly; and thereafter placing the assembly in a steam chamberwherein a differential external-internal pressure is imposed thereon andwherein the assembly is heated for only the limited period of timerequired to vulcanize the cushion gum layers and thereby permanentlyunite the components. A lugged tire 10 constructed in accordance withthe invention generally comprises, as shown in FIGS. 2 and 3, aconventional tire carcass 11 having a substantially smoothcircumferential surface 12 upon which a plurality of discrete and highlywear-resistant lugs 14, each fomred of precured retread rubber material14', are permanently secured by means of a thick intervening layer-likebody or mass 16 of relatively soft and flexible cushion gum material16'. The cushion gum mass 16 extends about the full length and width ofthe circumference 12 of tire carcass 10 and has a minimum thickness,preferably no less than 3/8 to 1/2 inch at any point, sufficient todissipate a significant part of the shearing forces imposed upon lugs 14during one of the tire. Cushion gum material 16' is readily curable suchthat the curing in situ of the mass 16 thereof upon carcass 11 can beand is rapidly effected without deterioration of or detriment to thetire carcass. As is indicated in FIGS. 7 and 8, when a lugged tire 10'is formed from a carcass 11' having a circumferential surface 12' whichis curved in a lateral direction, rather than being substantially flatas in the case of the carcass 11 of FIGS. 2 and 3, one or morestress-relieving and transversely-extending grooves 26 are providedwithin the outer surfaces of lug components 14.

Cushion gum material 16' and retread rubber material 14' are preferablyand respectively comprised primarily of natural rubber of very highgrade quality, such as that classified in the industry as Grade A, andof synthetic rubber, mixed in each case with additives such as fillers,vulcanizers, accelerators, anti-oxidants and the like to produce orenhance desired characteristics or properties in the respectivematerials. The desired properties possessed by cushion gum material 16'include particularly high tackiness, excellent bonding ability and arapid curing rate. Cushion gum material 16' also possesses goodflexibility before and after curing, and may be relatively soft, aDurometer hardness of approximately 60-61 being acceptable thereforafter curing. The lug components 14 formed of retread rubber material14' may be and preferably are somewhat harder than the cured cushion gummaterial 16', a Durometer hardness of approximately 64 or 65 beingacceptable therefor, and possess significantly greater density and cutand abrasion-resistance. The excellence of the aforesaid properties oflugs 14 is due not only to the composition of the retread rubbermaterial 14' from which they are formed, but also in more significantpart to the manner in which such material is cured.

Thus, as is diagrammatically shown in FIG. 4, lugs 14 are preferablyformed by molding retread rubber material 14' in a mold 18 having across-sectional shape, customarily generally rectangular (which term isintended to include trapozoidal) as shown, corresponding to the desiredcross-sectional shape of the lugs 14. Mold 18 has an open top throughwhich material may be inserted therein an dremoved therefrom, andpreferably has a length many times greater than the length of a singlelug 14. Mold 18 is received between upper and lower platens 20,20' of asuitable press having a length equal to or greater than that of the moldand a width preferably sufficient for a plurality of molds to bepositioned therein in side-by-side relationship to one another, ifdesired. One of the press platens, illustratively lower platen 20°, isverically movable toward and away from the upper platen as by a ram 22,and both platens are heated as by passage of steam or the like throughsuitable conduits 24 provided therein. Prior to the commencement of amolding operation, platens 20,20' and mold 18 are preheated by passingsteam through conduits 24 while the empty mold, which is formed of heatconductive metal, is disposed between the platens. Following preheatingof mold 18, the same is overfilled through its open top with retreadrubber material 14', and platen 20- is moved by ram 22 toward platen 20so as to subject material 14' to curing under high pressure andsubstantially uniformly applied heat. In the latter regard, it will beappreciated that heat is not only transmitted to material 14' from theupper platen 20 is direct engagement with its upper surface, but also istransmitted from both platens to the side and base surfaces of thematerial through the heat-conductive mold 18. Material 14' remainswithin mold 18, under a pressure preferably within the approximate range700 psi to 800 psi and a temperature preferably in the approximate rangeof 300° F to 350° F, until such material is fully and uniformly curedand has permanently assumed the shape of an elongate bar, a fragmentarylength of which is indicated by the numeral 14" in FIG. 4, of thedesired cross-sectional shape of the lugs 14 to be subsequently formedtherefrom. The required curing time will of course vary in accordancewith, among other things, the precise temperature and pressure employed,and the particular cross-sectional dimensions of the bar 14" which isformed. In the illustrative case of a bar having a width ofapproximately 41/2 inches and a thickness of approximately 21/2 inches,however, curing for approximately 1 hour at a temperature of 325° F anda pressure of 750 psi is acceptable. When molded and cured under theaforesaid conditions of high pressure and uniformly applied heat, theelongate bar 14" pressure and uniformly applied heat, the elongate heat14" possesses a more uniform density and markedly greater cut andabrasion-resistance that would be possessed by curing retread rubbermaterial in situ upon a tire carcass within a steam chamber.Additionally, the curing process may be and is effected relativelyrapidly, and without overcuring or undercuring of any part of thematerial.

After each elongate bar 14" of precured rubber material has been removedfrom its mold 18 and allowed to cool, it may be immediately severed atspaced locations along its length to form therefrom a plurality ofdiscrete lugs 14, to the base surface of which a layer 16" (FIG. 5) ofcushion gum material 16' may then be secured. Preferably, however, eachelongate bar 14" is simply placed in inventory at the location where itis to be subsequently used in tire building or rebuilding operations.This alternative is deemed preferable since it enables lugs 14 to be cutto the proper length for the building or rebuilding of any particularone of a plurality of tires of different widths, and thereby reduces theamount of materials which must be inventoried in order to constructlugged tires of various sizes. Additionally, while the bars 14" and lugs14 of precured retread rubber material have a practically indefinitestorage or "shelf" life, the readily-curable cushion gun material 16'may commence detrimental self-curing after approximately 90 days, evenat ambient temperatures. Preferably, therefore, cushion gum material 16'is not held in inventory for longer than approximately 90 days beforeuse thereof. At such time as cushion gum material 16 is assembled withlugs 14, the bases of the latter are first buffed or otherwise abradedto removed surface oxidation therefrom and to slightly roughen thealmost mirror-like finish produced thereon by the molding process usedin their formation. Layers 16" of cushion gum material 16' are thenpromptly applied to the lug bases by cementing and stitching techniques.The thickness of the cushion gum layer (FIG. 5) secured to the base ofeach lug 14 is preferably appproximately 3/32 inch. A removable coveringof cellophane film or the like is applied to the bottom of the cushiongum layer 16" upon each lug 14, to prevent oxidation thereof, unless thelug is to be immediately used in a tire building or rebuildingoperation.

When the tire 10 incorporates a used carcass 11, the circumferentialsurface 12 of the used carcass is first buffed or milled, as by use ofan apparatus such as disclosed in U.S. Pat. No. 3,646,984, to removetherefrom any contaminated surface rubber and remnants of the wornoriginal lugs thereon, and so as to render such surface substantiallysmooth and free from major protuberances or cavities. Following buffingsome indentations or cavities resulting from prior use of the carcass 11may still remain in its surface 12. If present, these are preferablyfilled, after buffing is completed, with cushion gum material 16'. Whenbuilding a lugged tire from a new carcass, there of course wouldnormally be no cavities to fill or original lugs to remove, but suchtire should still be buffed sufficiently to slightly roughen itscircumference and to insure that the same is substantially free fromsurface oxidation. While the circumference 12 of the thus-prepared newor used tire carcass 11 is substantially free from oxides, a layer 16'"of cushion gum material 16' is formed thereon and secured thereto. Thisis accomplished by applying suitable rubber cement, of which varioussatisfactory types are commercially available, to the circumferentialsurface 12 of tire carcass 11 which is to receive the previouslyprepared band of cushion gum material 16', then wrapping the band ofcushion gum material about the entire circumference of the surface 12,and thereafter stitching the cushion gum material 16' to surface 12 oftire carcass 11. Carcass 11 is preferably maintained in a slightlyoverinflated condition while cushion gum material 16' is secured asaforesaid thereto. This helps insure that any air entrapped betweencushion gum material 16' and surface 12 of carcass 11 during thecementing step will be expelled therefrom by and during stitching of thecomponents to one another. The cushion gum layer 16'" provided uponsurface 12 of carcass 11 overlies the full length and width of suchsurface and has a thickness of at least 3/8 to 1/2 inch. Layer 16'"might be formed from a single band of cushion gum material 16'possessing the requisite thickness and width, but would usually beformed by cementing and stitching a plurality of thinner and/or narrowerbands to each other and/or to surface 12 of carcass 11 to form thedesired layer 16'". Whether formed from one or a plurality of bands ofcushion gum material 16', layer 16'" may after its formation projectbeyond one or both sides of surface 12 of tire carcass 11. In such case,the layer may be trimmed along its side edge or edges to bring its widthinto substantial conformity with that of surface 12 or, alternativelyand as is sometimes desirable when employing a used tire carcass havingbadly worn shoulder portions, the excess width of the cushion gum layer16'" may be switched to such shoulder portions of the carcass.

Unless lugs 14 are to be promptly secured upon the cushion gum layer16'" formed upon carcass 11, such layer should be covered by a removablesheet of cellophane or like material to prevent oxidation of its exposedsurfaces.

Assuming that the retread rubber material 14' precured in mold 18 (FIG.4) was formed into and allowed to remain as an elongate bar 14", suchbar is now severed intermediate its length to form discrete lugs 14having longitudinal dimensions appropriate for their use upon thespecific tire being built or rebuilt. The particular length of lugs 14will of course vary in accordance with the width of the circumferentialsurface 12 of the tire carcass 11 employed, if the lugs are to extendcompletely across the tire as shown in FIGS. 3 and 4, and with thedesire angularity (if any) of such lugs relative to the tire's axis ofrotation. The bases of lugs 14 are buffed or otherwise abraded and thepreviously-described layers 16" (FIG. 5) of cushion gum material 16' arecemented and stitched thereto. Following removal therefrom of anyprotective sheets of cellophane material (not shown) or the likethereon, the lugs 14 carrying the base cushion gum layers 16" are thensecured, in desired spaced relationship and angularity relative to eachother, upon the cushion gum layer 16'" (FIG. 5) previously formed uponcircumferential surface 12 of tire carcass 11. The foregoing isaccomplished by cementing and stitching techniques similar to thoseemployed to secure layer 16'" to tire carcass 11, and while such carcassis maintained in an inflated and preferably slightly overinflatedcondition. Performance of the foregoing assembly steps is facilitated byreason of the fact that all of the components are in an unheatedcondition and are preferably at a temperature no greater than theambient temperature. If any of the components were in a heated conditionduring assembly, handling thereof would be more difficult and,additionally, premature partial vulcanization of the readily-curablecushion gum material 16' might undesirably occur.

Assuming that the circumferential surface of the tire carcass employedin the foregoing assembly is substantially flat, as in the case of thecarcass 11 of FIGS. 2 and 3, the assembly is next placed within asuitable steam chamber (not shown) wherein it is heated underdifferential external-internal pressure conditions for only the periodof time required to vulcanize the readily-curable cushion gum material16' and to thereby permanently unite the components. A steam-chambertemperature and pressure of approximately 265°-280° F and approximately80 psi effect the desired vulcanization of cushion gum material 16'without causing cracking or other deterioration of tire carcass 11, andare preferred. Automatic valving means (not shown) provided within thesteam chamber and communicating with the interior of carcass 11 removecondensation therefrom and maintain a differential, preferably in theapproximate range of 5 to 10 psi, between the pressure within thecarcass and the steam-chamber pressure. When the steam-chamber pressure.When the steam-chamber pressure is 80 psi, the aforesaid differentialmay conveniently be realized by carcass 11 being inflated to suchpressure when initially inserted into the steam chamber, and by thenhaving the automatic valving means so regulated as to release air fromcarcass 11 when the pressure therewith tends to rise above 84-90 psi dueto heating of the assembly within the chamber.

The aforesaid pressure differential, in conjunction with the stitchingforces previously applied to lugs 14 during their assembly with carcass11, united the layers 16", 16'" of cushion gum material 16' into thelayer-like mass 16 as shown in the FIG. 3. As in indicated in FIG. 3,some cushion gum material 16' is squeezed or extruded outwardly frombeneath each lug 14, forming rolls or beads adjacent its longitudinaledges, by the pressure-differential and stitching forces to which thelugs are subjected. However, since the combined original thicknesses ofthe layers 16", ≠'" beneath each lug 14 was considerably in excess of3/8 to 1/2 inch, the minimum thickness of mass 16 still is no less than3/8 to 1/2 inch notwithstanding the aforesaid extrusion, and the cushiongum mass normally and desirably will still be thicker (as shown in FIG.6) in the area immediately beneath lugs 14 than in the area between thelugs.

The time required for vulcanization of cushion gum mass 16 willnaturally vary to some extent with the size of the particular tire 10being built or rebuilt, but will in any event be significantly less thanthe time required for vulcanization if lugs 14 were not precured and/orif mass 16 were formed or retread rubber material rather than ofreadily-curable cushion gum material 16'. A heating time ofapproximately 4 to 5 hours is normally sufficient for a tire built inaccordance with the present invention, and is preferred, as compared toa heating time of 8 hours or more for lugged tires constructed inaccordance with the prior art.

Referring now to FIGS. 7 and 8, the lugged tire 10' there fragmentarilyshown incorporates a carcass 11' having a circumferential surface whichis curved, rather than flat, in the width direction thereof. Tire 10' isconstructed in the same manner as the previously described lugged tire10, except that one or more transversly-extending grooves 26 areprovided within the outer surfaces of its lug components 14 for thepurpose of alleviating which might be produced within the substantiallyflat lugs 14 by their forced comformity to curved circumferentialsurface or carcass 11'. Each groove 26 may be and preferably is formedafter lugs 14 have been assembled upon carcass 11', either before orafter treatment of the assembly within the steam chamber, by engagingthe outer surfaces of the lugs with a heated grooving iron or similartool (not shown) of generally U-shaped configuration while rotating thetire assembly about its axis. Each groove preferably has a depth ofapproximately one-half inch, and its bottom surface should be rounded orarcuate to prevent stress-concentrations thereat. Although threeequally-spaced grooves 26 are shown in FIGS. 7 and 8 within each lug 14,the number and spacing of the grooves may vary depending upon the degreeof curvature of the circumferential surface of the tire carcass and thethickness of the lugs employed.

The lugged tire of the present invention is exceedingly durable andlong-wearing. The composition and precuring, under high pressure anduniformly-applied heat, of lugs 14 causes these primary ground-engagingcomponents of the tire to be uniformly dense and strong, and exceedinglycut and abrasion-resistant. Additionally, lugs 14 do not tend to be tornaway from the tire carcass even under the impetus of the relatively highshearing forces and stresses exerted thereon during use. Such forces andstresses are believed to be dissipated in significant part by the thickcushion gum mass 16 interconnecting such lugs and the relatively-weakertire carcass. The possibility of failure of such carcass is alsominimized by the fact that the same is not subjected to excessiveheating or to any detrimental deformation during building of the tire.

While preferred embodiments of the invention have been specificallyshown and described, this was for purposes of illustration only, and notfor purposes of limitation, the scope of the invention being inaccordance with the following claims.

That which is claimed is:
 1. A method of building a vehicle tire of thelugged type, comprising:providing a tire carcass having a substantiallysmooth outer circumference free from major protuberances and cavities;providing a plurality of lug components of generally rectangular shape,each of said lugs being formed of precured retread rubber material andhaving substantially uniform strength and density and high cut andabrasion resistance; assembling upon said outer circumference of saidtire carcass said precured lugs and an intervening layer-like mass ofuncured and relatively soft cushion gum material having good bondingcharacteristics, a rapid curing rate, and greater flexibility in both anuncured and a cured condition than said retread rubber material of saidlugs; said layer-like mass of cushion gum material underlying said lugsand overlying substantially all of said circumference of said tirecarcass and having a minimum thickness of at least approximately 3/8 to1/2 inch; and thereafter heating the assembled cushion gum material,precured lugs and tire carcass while imposing a differentialexternal-internal pressure thereon, within a steam chamber to cure saidcushion gum material and permanently bond together all of saidcomponents of the assembly.
 2. A method as in claim 1, wherein saidassembling step includes securing layers of cushion gum material to saidouter circumference of said tire carcass and to the bases of said lugcomponents, and thereafter securing said lug components with base layersof cushion gum material thereon to the layer of cushion gum materialupon said outer circumference of said tire carcass.
 3. A method as inclaim 2, wherein said cushion gum layer secured to said circumference ofsaid tire carcass has a minimum thickness of at least approximatelythree-eights to one-half inch and overlies substantially all of saidtire carcass circumference.
 4. A method as in claim 3, wherein saidcushion gum layers secured to said bases of said precured lugs each havea thickness of at least approximately three-thirty-seconds inch.
 5. Amethod as in claim 4, and further including removing surface oxide fromsaid circumferential surface of said tire carcass and from said bases ofsaid precured lugs immediately prior to securing said cushion gummaterial thereto.
 6. A method as in claim 2, wherein said assemblingcomprises cementing and stitching said tire carcass, cushion gummaterial and precured lugs together while said components are in anunheated condition.
 7. A method as in claim 1, wherein the step ofproviding a tire carcass having a substantially smooth circumferenceincludes providing a used lugged tire and buffing the circumferencethereof to remove therefrom deteriorated surface rubber and remnants ofthe original lugs thereon.
 8. A method as in claim 1, and furtherincluding providing stress-relieving grooves within the outer surface ofsaid precured lugs.
 9. A method as in claim 8, wherein said grooves areprovided within said lugs subsequent to said assembling step.
 10. Amethod as in claim 9, wherein said grooves are formed by rotating saidassembly about its axis while engaging the outer surfaces of saidprecured lugs thereon with a heated groove-forming tool.
 11. A method asin claim 10, wherein said grooves have a depth of at least approximately1/2 inch and each have an innermost surface of arcuate configuration.12. A method as in claim 1, wherein the step of providing precured lugsincludes placing uncured retread rubber material within a mold,subjecting the uncured rubber material within the mold to heat andpressure to simultaneously cure said rubber material and mold the sameinto an elongate bar having substantially the desired cross-sectionalconfiguration of said lug components, and thereafter severing saidelongate bar at spaced intervals intermediate its length to provide saidlugs.
 13. A method as in claim 12, wherein said retread rubber materialis subjected within said mold to a pressure within the approximate rangeof 700 psi to 800 psi, and to a heating temperature in the approximaterange of 300°-400° F, and wherein the heat is applied to said retreadrubber material within said mold through more than one face thereof. 14.A method as in claim 12, including buffing the bases of said precuredlugs, and securing layers of said cushion gum material upon the base ofsaid precured lugs promptly after buffing thereof.
 15. A method as inclaim 11, including maintaining said tire carcass in an inflatedcondition during said assembling step.
 16. A method as in claim 1,wherein said pressure differential maintained during heating of saidassembly is at least approximately five psi.
 17. A method as in claim 1,wherein during heating of said assembly the internal pressure withinsaid tire carcass is raised to and maintained at approximately 5 to 10psi more than the steam pressure within said chamber.
 18. A method as inclaim 17, wherein during heating of said assembly the internal pressurewithin said tire carcass is raised to and maintained at approximately85-90 psi while the steam pressure within said chamber is raised to andmaintained at approximately 80 psi.
 19. A method as in claim 1, whereinsaid assembly is heated within said chamber at a temperature ofapproximately 265°-280° F for a time of no more than approximately 5 to6 hours.